This invention relates to continuously-variable ratio transmissions ("CVT's") of the toroidal-race rolling-traction type and especially to means for controlling the orientation of the rollers in the variators, that is to say the ratio-varying components, of such CVT's.
FIG. 1 is a simplified representation of part of one known variator, viewed in a direction perpendicular to the common axis N of the two discs G and J. A single roller A, which will in practice be one of a set of three disposed at equal angular intervals around the axis N, transmits traction between part-toroidal races F and H in discs G and J respectively, and is mounted within a carriage C to rotate about a center and axis (B) both of which are defined by and fixed relative to the carriage. A rod P connects the carriage C to a piston D which has freedom to move axially within a fixed cylinder E, and also to tilt slightly as it does so without losing seal.
Such a variator has been found to work efficiently in a so-called "torque-controlled" CVT where a pressure generated hydraulically (by means not shown) in cylinder E exerts a force on piston D, which for equilibrium must balance the reaction force resulting from the resultant torque at the contact between the roller A and the races F and H. Roller A changes its angle of orientation (or "tilt angle"), and thus the ratio it transmits between discs G and J, by tilting about the axis of rod P, and it has been found that each position of the center of piston D, within its range of axial movement within cylinder E, correlates with a unique tilt angle of the roller A. In other words, each equilibrium tilt angle of the roller is uniquely defined by just three points, namely the locations of contact of the roller A with races F and H and the location of the centre of the piston D. Such a variator, and the CVT of which it is part, is described and shown in more detail in Patent EP-B-0444086.
As is well known in this art, the center of the roller is at all times constrained to follow the center circle of the torus to which races F and H conform. That center circle must lie in the mid-plane M of the torus. Rod P, which as already noted defines the tilt axis of the roller, is inclined to that plane at an angle L, known as the castor angle. The advantages of operating a toroidal-race variator with a substantial degree of castor angle, say of the order of 15.degree., are well known in the art.
It will be seen that in the apparatus of FIG. 1, as is typical in the prior art, the axis of movement of the component which applies the reaction force (the piston D) and the axis of tilt (the rod P) coincide at the third of the three points by which each angular setting of the roller is uniquely defined. This coincidence imposes constraints upon the location and orientation of certain components, particularly the cylinder E, and thus on the overall dimensions of the variator. For example, because the axis of cylinder E is inclined to the transverse mid-plane M by the castor angle L, the radius at which the cylinder is located relative to the disc axis N exceeds the radius of the discs themselves. If it did not, a corner of the cylinder would be at risk of fouling the disc J.